Shielded high voltage electrical apparatus permitting potential test



Nov. 19, 1968 R. P. JOHNSTON 3,412,353

SHIELDED HIGH VOLTAGE ELECTRICAL APPARATUS PERMITTING POTENTIAL TEST Filed May 9, 1966 A c mud k/WWM United States Patent 3,412,353 SHIELDED HIGH VOLTAGE ELECTRICAL APPA- RATUS PERMITIING POTENTIAL TEST Richmond P. Johnston, Glenshaw, Pa., assignor to Allis- Chalmers Manufacturing Company, Milwaukee, Wis. Filed May 9, 1966, Ser. No. 548,621 13 Claims. (Cl. 336-105) ABSTRACT OF THE DISCLOSURE A ground level distribution transformer has a grounded metallic housing enclosing a core and coil assembly and provided with an aperture completely closed by aniinsulating member having a conductive electrode embedded therein opposite the aperture and connected to the transformer primary winding. The insulating member prevents accidental contact with the electrode and has aninstrument receiving recess opposite the aperture in which an instrument can be inserted to detect if the electrode is generating an electrostatic field, which would indicate that the transformer is energized.

' This invention relates to high potential electrical apparatus and in particular to shielded high potential electrical apparatus having means permitting a lineman to detect whether the apparatus is energized.

High voltage electrical apparatus such as power distribution system cables, transformers and voltage; regulators are a source of danger to linemen performing installation, maintenance or repair work thereon. Electrical transformers for residential underground distribution sys- 'tems are often installed in metallic ground level housings or in metallic tanks positioned in underground vaults, and shielded primary underground cables may be connected to the transformers through disconnectable plug-in connectors which are not load-break devices. Great hazards would exist for the operator and the apparatus might be heavily damaged if the separable connector was inadvertently disconnected while magnetizing or load current was flowing therethrough to the trans-former. The lineman must be certain that the transformer is not hot, i.e., energized, before he opens the connector or enters the vault or housing to perform work thereon, and he must also be certain that both the cable and transformer are deenergized before he mates the male and female portions of the connector. When the male and female connector elements are separated, the lineman can attempt to draw. a fuzzing arc with a Hotstick to determine if the elements of the power'syste'm are hot. However, when the male and female connector elements are mated, the metallic tank of the transformer and the conductive shield on the cable and the disconnectable connector form a continuous outer sheath which is usually grounded and prevents determination of whether potential exists in the electrical apparatus.

It is an. object of the invention to provide a shielded high voltage electrical apparatus which readily permits the lineman to determine whether the apparatus is energized.

This and other objects and-advantages of the invention will be more readily apparent from the following detailed description when considered in conjunction with the accompanying drawing wherein:

FIG. 1 is a schematic plan view of a ground level distribution transformer embodying the invention;

FIG. 2 is a sectional view through the means which permit a lineman to determine if the apparatus of FIG. 1 is energized; and

FIG. 3 is a sectional view through an alternative embodiment of the means shown in FIG 2.

The invention may be incorporated in any element of a shielded underground power distribution system such as the transformer, the disconnectable connector of the primary underground cable, but the invention will be described with reference to a distribution transformer.

Referring to FIG. 1 of the drawing, a ground level distribution transformer shown schematically in plan view comprises a grounded metallic housing 10 having a sealed compartment 11 filled with insulating dielectric such as transformer oil 12. A transformer core and coil assembly immersed in oil 12 includes a magnetic core 14 linked by a primary winding 15 and a secondary winding 16. The ends of secondary winding 16 may be connected by leads 17 to axial conductor studs on secondary insulating bushings 18 which;are mounted on a vertical panel 19 of metallic housing 10 and extend into the low voltage compartment 20 of the housing 10. A barrier 21 divides the low voltage compartment 20 from the high voltage compartment 22. One end of the primary winding 15 may be connected to the grounded metallic housing 10, and the other end of primary Winding 15 may be connected by a lead 23 to the axially extending conductor stud of a primary insulating bushing 24 that mates with a disconnectable connector 25 which is shown as being of the elbow type. The connector 25 is well known and commercially available'and may include an axial conductor 26 surrounded by insulation and an outer conductive sheath and having a female terminal 29 one end of which releasably engages a complementary male terminal 30 member of insulating bushing 24. The other end of axial conductor 26 may be connected through stress grading means (not shown) to the axial conductor of an underground cable 31 which extends through a conduit 32 in the concrete pad on which the housing 10 is supported. The outer sheaths on cable 31 and connector 24 may be grounded and connected to grounded metallic housing 10 and form a continuous conductive shield which prevents a lineman from using a Hotstick to attempt to draw a fuzzing arc to determine if the electrical apparatus is energized.

As shown in FIG. 2, an aperture 33 in panel 19 of metallic housing 10 may receive an elongated cylindrical insulating member 34 having a conductive electrode 35 embedded therein and an outwardly extending circumferential flange 36 disposed against the portion of panel 19 defining aperture 33. A metallic clamping plate 37 extending above circumferential flange 36 may be aflixed by bolts 38 to metallic studs 39 welded to panel 19 to secure insulating member 34 to metallic housing 10. A continuous resilient gasket 40 is compressed within a circumferential groove 41 in flange 36 to hermetically seal metallic housing 10 at aperture 33. Electrode 35 may be connected by a conductor 42 to one end of primary winding 15, and as shown in FIG. 2, electrode 35 is threaded and conductor 43 is crimped to an eye terminal 43 secured to electrode 35 by a nut 44.

An axial instrument receiving recess, or well 46, provided in insulating member 34 is accessible from the exterior of metallic housing 10, and electrode 35 extends to adjacent the bottom of recess 46 so that a relatively thin wall 47 of insulating material exists between the bottom of recess 46 and electrode 35 which is of suflicient thickness to assure safety of the lineman when an electrostatic field detecting instrument is inserted into recess 46. Metallic housing 10 forms an outer conductive shield which normally prevents detection of the electrostatic field generated by the transformer, but electrode 35 is connected to the transformer primary winding 15 and is disposed opposite aperture 33, thus permitting the electrostatic field generated by electrode 33 to be detected by any suitable electrostatic field detecting instrument 50 held opposite aperture 33. The electrostatic field detecting instrument may be of the type commercially available under the trademark Statis'cope which'isapproximately the size of a fountain pen and includes an insulating casing enclosing a neon bulb 51 and having a metallic probe 52 at one end electrically connected to one terminal of neon light 51 and a metallic spring 44 engaging the other terminal of the neon light and urging it toward the probe 52 and wherein the electrical current for lighting the neon light 51 flows to earth through the capacitance of the lineman holding the detecting instrument.

A metallic cover 56 normally closes the instrument receiving well 46 and is releasably secured by screws 57 engaged in tapped inserts 58 molded within flange 36 of insulating member 34. Cover 56 engages clamping plate 37 and thus is normally grounded to assure safety of a lineman coming into contact therewith. Removal of cover 56 exposes instrument receiving well 46 and permits the lineman to determine if the electrical apparatus is energized.

The circumferential flange of the FIG. 2 embodiment is omitted from the insulating member 34' of the embodiment of FIG. 3, and a metallic tubular support member 60 having external threads thereon is embedded in the 2. In the combination defined by 'claim'l, whereinsaid sheath is grounded.

3. In the combination defined by claim 2 wherein said electrode is embedded within said insulating member and extends adjacent to the surface of said insulating member.

4. In the combinationdefined by claim 3 wherein said insulating member has an instrument guiding recess therein opposite said aperture and said electrode and accessible from the exterior of said apparatus.

5. In'the combination defined by claim 4 and including a protective conductive cover, and means releasably securing said cover in a position wherein it closes said instrument guiding recess and electrically engages said material of insulating member 34' and is welded to the margin of panel 19 defining aperture 33. A cup shaped metallic cover 61, which may be a standard pipe cap, has internal threads 62 which releasably engage the external threads on tubular support member 60 to normally prevent access to instrument receiving well 46 and is thus electrically connected to the metallic housing and grounded. Cover 61 may have an eye 63 afiixed thereto to facilitate removal with a Hotstick.

The preferred embodiment has been described with reference to a transformer, but the invention is not so limited and also may be embodied in other shielded high voltage apparatus such as cables, disconnectable connectors, voltage regulators or separately housed sectionalizing switches. Many modifications and variations of the invention will be apparent to those skilled in the art, and consequently it is intended in the appended claims to cover all such modifications and variations which are within the true spirit and scope of the invention.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In combination, high potential electrical apparatus having an outer conducting sheath provided with an aperture therein,

an insulating member closing said aperture,

and a conductive electrode electrically connected to and positioned within said high potential electrical apparatus opposite said aperture and being covered by said insulating member,

said insulating member completely closing said aperture so that said electrode is not exposed at the exterior of said apparatus and cannot he accidentally contacted and said insulating member being penetrable by the electrostatic field generated by said electrode when said apparatus is energized whereby said electrostatic field penetrates said aperture and said insulating member and can be detected from the exterior of said apparatus by an electrostatic field detecting instrument to determine if said apparatus is energized.

sheath.

6. In the combination defined by claim 2 wherein said conducting sheath is a metallic casing.

7. In the combination defined by claim 6 wherein said electrical apparatus is an electrical transformer within said casing having primary and secondary windings and said electrode is connected to said primary winding.

8. In the combination defined by claim 6wherein said insulating member is an elongated cylinder extending through said aperture and having an axial instrument guiding recess therein accessible from the exterior of said casing and a wall of insulating material separating said electrode from the bottom of said recess.

9. In the combination defined by claim 8 and including a metallic cover, and means releasably securing said cover in a position wherein it closes said recess and electrically connects said cover to said metallic casing.

10. In the combination defined byclaim 8 and including a tubular metallic member coaxial with and embedded in said insulating member in surrounding relation to said recess and having the outer periphery thereof secured to the portion of said casing defining said aperture.

11. In the combination defined by claim 10 wherein said tubular member is threaded and including a cup shaped metallic cover having threads in the sidewall portion thereof releasably engaging said threads on said tubular member and normally preventing access to said instrument receiving recess.

12. In the combination defined by claim 6 wherein said insulating member is an elongated cylinder and extends through said aperture and has a circumferential flange disposed against the portion of said casing defining said aperture and said electrode extends axially of said cylindrical member.

13. In the combination defined by claim 12 wherein said circumferential flange has a circumferential groove therein opposite said metallic casing and including means for clamping said flange against said casing and a continuous resilient gasket within said groove compressed between said casing and said flange and hermetically seale ing said casing at said aperture.

References Cited UNITED STATES PATENTS LARAMIE E. ASKIN, Primary Examiner. 

